The invention relates to an identification system comprising a plurality of transponders and an interrogator, to the transponders and interrogators themselves and to a method of identifying a plurality of transponders through an interrogation process. The invention further relates to method and apparatus for improving the identification systems disclosed in EP 494,114 A and EP 585,132 A.
Identification systems are known in which a plurality of transponders are activated by a power signal (or an xe2x80x9cinterrogation signalxe2x80x9d) and then transmit reply signals, usually containing identification data, to a receiver, which forms part of the interrogator. The signals may be transmitted in many ways, including electromagnetic energy, e.g. radio frequency (RF), infrared (IR), and coherent light, and sound, e.g. ultrasound. For example, the transmission may be achieved by actual emission of RF energy by the transponders, or by the modulation of the reflectivity of an antenna of the transponder, resulting in varying amounts of RF energy in the interrogation signal being reflected or back-scattered from the transponder antenna.
A difficulty with such systems is that with a plurality of transponders present, clashes can occur between transmission of different transponders. In general, if the transmissions of two transponders overlap or clash, the transmissions are polluted and therefore lost, since the receiver cannot distinguish the separate transmissions (see FIG. 1). One solution is to provide for each transponder to transmit repeatedly until its entire transmission is successfully received by the interrogator. Various approaches have been taken to addressing this problem, with varying success.
GB 2,116,808 A discloses an identification system in which the individual transponders are programmed to retransmit data in a pseudo-random manner. Timing signals for the transponders in this identification system are derived from a crystal oscillator, thereby making the transponders expensive to manufacture.
EP 467,036 A describes another identification system which uses a pseudo-random delay between transponder data transmissions. In this example, a linear recursive sequence generator is seeded by the transponder identification address to make the pseudo-random delay as random as possible.
EP 161799 A discloses an interrogator/transponder system in which each transponder transmits a reply signal consisting of a uniquely coded identification number. The interrogator then re-transmits the signal it has received and each transponder decodes the signal and checks the data against it""s own identification number. In the event that a particular transponder recognizes it""s own code, that transponder discontinues the reply signal or adjusts to receive further instructions (all others having shut down). If interference occurs because two or more transponders are transmitting at the same time, the interrogator waits until a valid signal is received.
The systems described in the above which use a random or pseudo-random timing rely on the creation of random or pseudo-random delays in the generation of response signals to ensure that all transponders will eventually be identified by the interrogator. However in some circumstances the frequency of the clashes between transponders is sufficiently high for the number of successful identifications to be drastically reduced, and the overall time taken to identify all the transponders to be undesirably extended.
According to the present invention there is provided a method, an identification system, a transponder and an integrated circuit as defined by the appended claims.
According to one aspect of the invention there is provided a method of identifying a plurality of transponders in an interrogation comprising transmitting an interrogation signal to the transponders and receiving response signals from the transponders, each transponder being capable of transmitting the response signal at any time during a waiting period, characterised in that each transponder may alter the maximum length of the waiting period during the interrogation process.
According to another aspect of the invention there is provided an identification system comprising an interrogator and a plurality of transponders, the interrogator including a transmitter for transmitting an interrogation signal to the transponders, each transponder including a receiver for receiving the interrogation signal, a transmitter for transmitting a response signal after receipt of the interrogation signal and means for generating the response signal at any time during a waiting period characterised in that each transponder has means for altering the maximum length of the waiting period during interrogation of the transponders by the interrogator.
The transponders may alter the maximum length of the waiting period (the maximum wait time) during the interrogation. If there are too many transponders in the interrogation field and the entire system is heavily congested, the maximum wait time may be increased to increase the chances of an individual transponder transmitting during a quiet time. Conversely, if there are very few transponders in the interrogation field and there are very large quiet periods, the maximum wait time may be reduced to improve the speed of interrogation.
The transponders may dynamically alter the maximum wait time in response to an instruction from the interrogator. Alternatively, the transponders may be adapted to detect either heavy congestion or large quiet periods and alter the maximum wait time accordingly.
The maximum wait time for a group of transponders may be optimized in an initial interrogation, and used, in subsequent interrogations or the appropriate maximum wait time may be determined during an optimisation phase. The transponders may alter the maximum wait time in response to an instruction from the interrogator or in response to external conditions present during the interrogation.
If an instruction from the interrogator is used to alter the maximum wait time, these instructions may take the form of a modulation or other modification of the interrogation signal. Alternatively, these instructions may take the form of separate signals transmitted by the interrogator, for example at a frequency or frequencies which differ from the interrogation signal.
The transponders may increment or decrement the maximum wait time in a number of stages or alter to any length, e.g. number of clock cycles or elapsed time.
Where the transponders are not xe2x80x9cswitched-offxe2x80x9d or otherwise disabled after successful identification and are adapted to retransmit the response signal after the waiting period, the waiting period may be altered after successful identification. Further, when repeated successful identification of a transponder occurs the waiting period may be altered, for example, by repeatedly doubling the period.
It will be appreciated that the alteration of the waiting period impose no limitations on the technique used for determining the actual waiting period employed by a transponder in a particular circumstance. In particular the random and pseudo random transmission and retransmission techniques previously referred to can be used in conjunction with the invention and its preferred features.
Transponders and interrogators, such as those described in EP 494,114 A and EP 585,132A (the entire contents of which are incorporated herein by reference), may be adapted to produce transponders and interrogators according to the invention. The transponders may be uniquely coded, or several transponders may share the same code.